Preheat an outdoor grill for medium-high heat and lightly oil the grate. Whisk oil, salt, and black pepper together in a bowl.

Remove the tough bottom leaves from each cauliflower, taking care not to remove the stem. Place the cauliflower heads onto a cutting board with the stem side facing upwards; cut vertically on both sides of the stem to remove the loose florets. Cut each cauliflower vertically through the stem into 2 even "steaks", 1 to 1 ½ inches thick; remove any remaining small leaves from each steak. Place the 4 steaks on a baking sheet.

Brush one side of the steaks with some of the oil mixture.

Place steaks on the grill, oiled-side down; grill for 5 minutes. Brush more oil mixture on top and gently flip over; grill until golden brown and slightly charred, 5 to 7 minutes more. Remove to a platter and drizzle with truffle oil. Sprinkle with Parmesan cheese and parsley.